Florida, Caribbean beaches could be choked out by record levels of Sargassum seaweed this summer

Drone video: Seaweed covers some South Florida beaches

Drone video taken on Wednesday showed clumps of seaweed covering parts of South Florida’s shoreline.

TAMPA, Fla. – As summer approaches, beachgoers may need to check an additional forecast detail before heading to the shore: the Sargassum seaweed situation.

This year is already a record for the amount of Sargassum in the central Atlantic Ocean. 

"What we observed in April was far and away greater than anything the satellites have recorded over the last 25 years and presumably ever as far as what would be expected," University of South Florida Research Assistant Professor Brian Barnes said.

Barnes is part of the small team using satellites to monitor Sargassum Seaweed. USF’s College of Marine Science Optical Oceanography Laboratory maintains the Sargassum Watch System, an outlook on current and future seaweed blooms in the Caribbean Sea and Gulf, using a suite of satellite data from NASA, NOAA and private satellites.

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The outlooks and forecasts from USF have become increasingly important each year because massive amounts of Sargassum began washing up on the shores of Florida’s east coast and the Caribbean in 2011. It’s a relatively new climate phenomenon that is not fully understood.

A pile of sargassum, or seaweed, on Key Biscayne's coastline on Thursday, April 24. (Ashley Miznazi/The Miami Herald/Tribune News Service via Getty Images)

(Getty Images)

Total amounts of seaweed in the eastern Caribbean Sea and the western Atlantic reached "surprisingly high levels," according to the USF Sargassum Watch System. Both were 200% higher than their historical records for the month, and the combined regions were 150% higher than the historical record for April. 

Prior to satellite data, a bloom of this size would have made headlines.

"Somebody would have noticed," Barnes said.

"And we're not yet to the peak time of the Sargassum, which is usually June. So there's still a ton of biomass that's kind of not really affecting anyone. It's offshore in the tropical Atlantic right now, but a lot of it is migrating into the Caribbean and there's a ton inside the Caribbean," Barnes said. "Some of that will eventually make its way out and affect more US coastlines."

While this is a new issue, some seaweed has always been there, just not in the masses we are seeing now arriving at beaches, creating a problematic issue for coastal tourism as the decomposing seaweed releases a stench best described as rotten eggs.  

The whole Atlantic contains 31 million metric tons of Sargassum spread over a large area. The impacts on beaches in the Caribbean and Florida can be patchy, said Barnes. Florida’s West Coast won’t see major impacts because of how the ocean currents flow. 

A map showing the average Sargassum seaweed for the month of April 2025 with warm colors representing a higher amount in the Caribbean Sea and Gulf. 

(Optical Oceanography Lab at the USF College of Marine Science / FOX Weather)

Heading into Memorial Day weekend, there is a steady amount of sargassum in the Gulf region, about 200,000 metric tons – a fraction compared to the overall Atlantic Sargassum Seaweed belt. With the right combination of winds, currents and weather, a giant blob of Sargassum can end up on a Florida beach along the Atlantic coast. Local impacts are hard to predict more than a week in advance.

"A beach may get impacted, whereas a nearby beach would have absolutely nothing," Barnes said. "So we are working towards, and this is something that it's coming online as this season, fortuitously it's come online as the season is a pretty bad sargassum season, is that we've got some of these higher resolution, the Sentinel-2 and other data sets that provide, for which we can see patches in that near shore area, that provide us a little bit more clarity on where."

Barnes and the Sargassum Watch System team are working with additional NOAA funding this year to expand their forecasting ability. The Watch System now includes all of the U.S. waters impacted by the Sargassum belt, including the Florida Keys and eastern Florida, the U.S. Virgin Islands and Puerto Rico.

What happens to all that seaweed?

South Florida beach communities are removing it using industrial equipment as quickly as they can ahead of the summer rush, while others have looked at the biomass as a potential business opportunity. 

Barnes said he is contacted nearly every day by startups that want to use seaweed for different purposes.

Swaths of sargassum, or seaweed, washed up in Key Biscayne on Thursday, April 24 and was raked into piles to be taken to the landfill. (Ashley Miznazi/The Miami Herald/Tribune News Service via Getty Images

(Getty Images)

"There is biomass there that's coming ashore that can have value for cosmetics and pharmaceuticals and construction materials and biofuels … but you need to get to it quickly, meaning you have to have your stuff staged," Barnes said. "And if there are marine resources that need to be protected, setting out barriers or at least staging equipment to kind of protect the critical infrastructure, critical resources, is the reason to make that step and make some forecast of where a particular inundation might happen."

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Barnes said the seaweed forecasting outlook is attracting attention from many different stakeholders, including fisheries, tourism groups, local governments and even the Mexican Navy. 

Still, for average beachgoers, seaweed is mostly an annoyance and not a major health threat. Some with respiratory issues may have an adverse response, but it's not as severe as red tide, another issue plaguing Florida's coastal communities. 

There is also a concern about arsenic in the seaweed tissue, so it's not advised to handle it. 

"For most people, it's just smelly," Barnes said.